1. Field of the invention
The present invention relates to a turbocharger for an internal combustion engine, and more particularly, to an improvement of a compressor housing thereof.
2. Description of the Related Art
A turbocharger for an internal combustion engine receives exhaust gas from an engine exhaust pipe, rotationally drives a turbine wheel in a turbine housing, compresses air within a compressor housing under the action of rotation of compressor impellers arranged via a drive shaft integrally formed with the turbine wheel, and supplies the compressed air to the engine. The compressor housing and the compressor impeller in the turbocharger as described above are generally made of aluminum alloy castings.
An engine with a turbocharger is now demanded to have a supercharging effect from a low-revolution region of the engine. In the turbocharger, making an outside diameter of a curved profile portion of the compressor impeller and a gap formed between the profile portion and the inner wall surface of the compressor housing corresponding thereto as small as possible while improving a blade-shape of the compressor impeller is favorable for improving efficiency of the compressor. However, the small gap involves a risk that the curved profile portion of the compressor impeller rotating at an extra-high velocity may come into contact with the inner wall surface of the compressor housing due to slight shaft vibration, resulting in breakage of the impeller, or further in destruction of the drive shaft.
In a conventional turbocharger, therefore, it has been the usual practice to provide a gap of from about 0.3 mm to 0.5 mm between the inner wall surface of the compressor housing and the curved profile portion of the compressor impeller.
Making the gap between the impeller and the housing as small as possible by a thermal-spray coating provided in the housing is already known, for example, for a gas turbine (as disclosed in JP-B2-50-690, JP-A-52-72335, and JP-A-52-85031). More recently, JP-B2-04-40559 proposes a method of, in a turbocharger for automobile, forming by thermal spray a resin coating comprising a mixture of soft metal and resin or graphite onto the inner wall surface of a compressor housing as a means of making the above gap small and preventing occurrence of a damage to the compressor impeller even upon contact with the compressor impeller.
As a means of making the gap between the compressor impeller and the housing in a turbocharger of an internal combustion engine small, and preventing occurrence of a damage to the impeller even upon contact with the compressor impeller, JP-A-06-307250 proposes a turbocharger in which a wall member separately formed from a composite material comprising a resin such as PTFE (polytetrafluoroethylene) or a mixture of the resin and graphite or glass wool is attached onto a wall surface of at least the portion of the compressor housing wall surface corresponding to a curved profile portion of the compressor impeller.
In the conventional art, the presence of a necessary minimum gap T within a range of from 0.3 to 0.5 mm between the curved profile portion of the compressor impeller and the inner wall surface of the compressor housing puts restriction on improvement of compressor efficiency.
A thermal spray coating technique recently proposed, on the other hand, while being effective for improving compressor efficiency, needs making a consideration in productivity with respect to thermal spray equipment, capability to handle many different types of compressors and masking of products, and thus the problem is that a product cost is higher.
Even when the thermal spray coating technique is replaced by a technique for improving compressor efficiency, in which a separately formed resin member is attached to the wall surface and a gap between the compressor housing inner wall surface and the curved profile portion of the compressor impeller of a turbocharger is made small, it is important to rotate the compressor impeller at extra-high velocity without damaging the compressor impeller upon contact between the wall member and the compressor impeller. That is, it is important, upon contact of these members, to smoothly shave the compressor housing wall member without causing any damage such as deformation or breakage to the compressor impeller.